*2.5. Validation*

To ensure the stability of the numerical solution, the sphere tests of Charters and Thomas [26] were chosen for validation. Figure 5 shows a comparison of the drag coefficient obtained from our simulations and the work of Charters and Thomas, in which experiments were conducted with small spheres at various velocities. A 2D axisymmetric sphere was simulated with an unsteady moving mesh. The numerical method used for the validation was the same as those mentioned in Section 2.4. Figure 5 shows a similar trend in the drag coefficient between the two studies, indicating a good agreement. Therefore, our selected numerical method is appropriate to simulate the moving pod–tube configuration.

**Figure 5.** Comparison of drag coefficients between the 2D axisymmetric simulation and the experimental data for a 14.28-mm sphere reported by Charters and Thomas [26].
